There are at least five legal and political issues arising out of Snowden's revelations on which reasonable opinion is divided. These include whether Snowden should enjoy the legal protection accorded a whistleblower who reveals wrongdoing; whether his revelations have weakened the counter-terrorism apparatus of the US or the UK; whether, conversely, they show the need for an overhaul of surveillance powers on both sides of the Atlantic (and even an international agreement to protect partners like Germany); whether parliament has been misled by the services about the extent of intrusive surveillance; and whether the current system for parliamentary oversight of the intelligence and security services is sufficiently robust to meet theinternational standards laid down by my predecessor at the UN, Martin Scheinin.

These questions are too important for the UN to ignore, and so on Tuesday I am launching an investigation that will culminate in a series of recommendations to the UN general assembly next autumn. As in the case of Chelsea Manning, there are also serious questions about sensitive information being freely available to so many people. The information Snowden had access to, which included top-secret UK intelligence documents, was available to more than 850,000 people, including Snowden – a contractor not even employed by the US government.

There is, however, one issue on which I do not think reasonable people can differ, and that is the importance of the role of responsible media in exposing questions of public interest. I have studied all the published stories that explain how new technology is leading to the mass collection and analysis of phone, email, social media and text message data; how the relationship between intelligence services and technology and telecoms companies is open to abuse; and how technological capabilities have moved ahead of the law. These issues are at the apex of public interest concerns. They are even more important – dare I say it – than whether Hugh Grant's mobile was hacked by a tabloid.

The astonishing suggestion that this sort of journalism can be equated with aiding and abetting terrorism needs to be scotched decisively. Attacking the Guardian is an attempt to do the bidding of the services themselves, by distracting attention from the real issues. It is the role of a free press to hold governments to account, and yet there have even been outrageous suggestions from some Conservative MPs that theGuardian should face a criminal investigation.

It is disheartening to see some tabloids give prominence to this nonsense. When the Mail on Sunday took the decision to publish the revelations of the former MI5 officer David Shayler, no one suggested that the paper should face prosecution. Indeed, when the police later tried to seize the Guardian's notes of its own interviews with Shayler, Lord Judge, the former lord chief justice, refused to allow it to happen – saying, rightly, that it would interfere with the vital role played by the media to expose public wrongdoing.

When it comes to damaging national security, comparisons between the two cases are telling. The Guardian has revealed that there is an extensive programme of mass surveillance that potentially affects every one of us, while being assiduous in avoiding the revelation of any name or detail that could put sources at risk. Rusbridger himself has made most of these decisions, as befits their importance. The Mail on Sunday, on the other hand, published material that was of less obvious public interest.

An even closer example is Katharine Gunn, the GCHQ whistleblowerwho revealed in 2003 that the US and UK were spying on the missions of Mexico and five other countries at the UN, in order to manipulate a vote in the security council in favour of military intervention in Iraq. Like Snowden, her defence was that she was acting to prevent a greater wrong – the attempt to twist the security council to the bellicose will of the US and UK. She was charged under the Official Secrets Act, but the case was dropped because the director of public prosecutions and attorney general rightly concluded that no jury would convict Gunn.

There can be no doubt that the Guardian's revelations concern matters of international public interest. There is already an intense debate that has drawn interventions from some of the UK's most senior political figures. Wholesale reviews have been mooted by President Obama, Chancellor Merkel and Nick Clegg, Britain's deputy prime minister. Current and former privy councillors and at least one former law officer have weighed in.

In the US, a number of the revelations have already resulted in legislation. Senior members of Congress have informed the Guardian that they consider the legislation to have been misused, and the chair of the US Senate intelligence committee has said that as a result of the revelations it is now "abundantly clear that a total review of all intelligence programmes is necessary".

In Europe, and particularly in Germany (which has a long and unhappy history of abusive state surveillance) the political class is incandescant. In November the Council of Europe parliamentary assembly endorsedthe Tshwane International Principles on National Security and the Right to Information, which provide the strongest protection for public interest journalism deriving from whistleblowers. Lord Carlile, the former independent reviewer of terrorism legislation in the UK, took part in the drafting of the principles and has endorsed them as an international template for resolving issues such as the present one. Many states have registered serious objections at the UN about spying, and there are diplomatic moves towards an international agreement to restrict surveillance activity. In direct response to the Guardian's revelations, Frank La Rue, the special rapporteur on freedom of expression, has brought forward new guidelines on internet privacy, which were adopted last week by the UN general assembly.

When it comes to assessing the balance that must be struck between maintaining secrecy and exposing information in the public interest there are often borderline cases. This isn't one. It's a no-brainer. The Guardian's revelations are precisely the sort of information that a free press is supposed to reveal.

The claims made that the Guardian has threatened national security need to be subjected to penetrating scrutiny. I will be seeking a far more detailed explanation than the security chiefs gave the intelligence committee. If they wish to pursue an agenda of unqualified secrecy, then they are swimming against the international tide. They must justify some of the claims they have made in public, because, as matters stand, I have seen nothing in the Guardian articles that could be a risk to national security. In this instance the balance of public interest is clear.